Fast wave crossed field travelingwave tube



Feb. 21, 1967 K W. FRlz 3,305,752

FAST WAVE cRossED FIELD TRAVELING-WAVE TUBE Filed Dec. 6, 1963 INVENTOR.Fifi wm rn? me z 4 IL w BY 4.4L.

24@ zs/Wv- E "L S-4 fam@ United States Patent() 3,305,752 FAST WAVECROSSED FIELD TRAVELING- j WAVE TUBE Walter Friz, Greene County, Ohio,assignor to the United States of America as represented by the Secretaryof the Air Force Filed Dec. 6, 1963, Ser. No. 328,763 1 Claim. (Cl.315-39) structure is a rectangular waveguide Wound in a linear v spiraland having a slit along its plane of symmetry for the passageof adisc-shaped electron beam for interacting with the wave propagated inthe guide. The purpose of this invention is t provide a traveling-wavetube of the above type S0 constructed that the phase relation betweenthe propagated wave and the electron beam modulation along any radius ofthe beam is such that a transfer of energy occurs from the beam to thewave. In accordance with the invention, this is accomplished bydecreasing the larger dimension of the waveguide as the radius of thespiral increases in order to increase the phase velocity of thepropagated wave as required to maintain the proper phase relationbetween the wave and the beam modulation.

The invention will be described in more detail with reference to thespecific embodiment thereof shown in the accompanying drawings in whichFIGS. 1 and 2 are cross sectional views of a travelingwave tubeincorporating the invention,

FIG. 3 shows the electrical connections to the electrodes of the tube inFIGS. 1 and 2, and

FIG. 4 illustrates a method of coupling to the spiral waveguide.

Referring to FIGS. 1 and 2, the traveling wave tube is contained in anannular housing of 4magnetic material having an upper half and a lowerhalf 11. Centrally located in these halves are magnetic pole pieces 12and 13 which extend toward each other, leaving space between their areden ds for an annular concave cathode 14. Series connected magnet coils15 and 16 surround the pole pieces 12 and are supplied with directcurrent from a suitable source (not shown). The coil winding directionsare such that the ploe pieces from similar magnetic poles, north polesfor instance, so that they magnetic flux passes radially outward fromthe cathode of the tube parallel to the plane of symmetry 17 of the tubeto the outer rims of the upper and lower halves 10 and 11 of thehousing, and thence equally through-the upper and lower halves back tothe pole pieces. The concave cathode 14 together with the acceleratingelectrodes 18 and 19 act as an annular electron gun causing theelectrons emitted by the cathode to converge into a thin discshaped beam20 centered on the plane of symmetry 17 and extending to collectorelectrode 21, the electrons being constrained to follow paths generallyradial and parallel to the plane of symmetry by the above describedmagnetic field. The electron beam system power supply circuit is shownschematically in FIG. 3. The cathode may be insulated and supportedbetween the pole pieces 12 and 13 in any suitable manner.

The traveling-wave structure of the tube consists of a rectangularwaveguide, having a greater dimension a and a lesser dimension b, woundin a linear spiral defined by the equation 3,305,752 Y Patented Feb. 21,1967 where R0 is the starting radius, 6 is measured in radians and b-j-sas shown in FIG. 2.

This structure may be formed in two halves 22 and 23 joined at plane 17to form a structure symmetrical with respect to this plane. Thewaveguide walls in each half of the structure are made of such lengthas, when the structure is assembled, to leave a slit at the plane lofsymmetry 17 for passage of electron beam 20. The two halves 22 and 23are joined to each other and to collector electrode 21 at theircircumferences and t0 pole pieces 12 and 13 at their centers bysoldering, brazing, welding or other suitable method providing gas-tightjoints so that the traveling wave structure may be evacuated.

The spiral waveguide may be coupled to external circuits by waveguides24 and 25 which may be joined to the spiral waveguide in the mannerillustrated in FIG. 4. In each case, a gas-tight window 26 is providedto seal the interior of the traveling-wave structure.

The rectangular waveguide is operated in its dominant mode, the TELOmode. In this mode the electric field is transverse to the direction ofpropagation and its lines are normal to the larger dimension lof thewaveguide with maximum intensity at the center. The electric eld linesare therefore parallel to the electron beam and densest on either sideof the beam. The lines of the magnetic ield form closed loops parallelto the wider surface of the waveguide.

In order for the electron beam and the propagated wave in atraveling-wave tube to interact in such a way that the wave energy isincreased, it is necessary that the phase velocity of theelectromagnetic wave be approximately equal to the average velocity ofthe electrons. Applying this requirement to a tube of the type shown inFIG. 2, in order for this condition to obtain along any radial filamentof the beam, such as the filament along line 27 in FIG. 2, it isnecessary that the beam see substantially the same phase of theelectromagnetic wave in each successive transit of the waveguide alongline 27. With a constant phase velocity in the guide this would beimpossible except along certain radial filaments of the beam because ofthe increasing lengths of the spiral turns. In accordance with theinvention, the' phase velocity is increased by a continuous reduction ofthe dimension a of the guide as required t-o provide the proper phaserelation 4along all radii of the disc beam. The required value of a atany point in the spiral may be determined as follows:

vIn order that the phase relation between the electromagnetic -wave andthe beam density modulation wave, which results from the velocitymodulation of the beam electrons by the electric field in the waveguide,shall remain substantially the same, the phase shift encountered by thewave in one spiral turn should differ from the phase shift encounteredby the beam modulation wave in traversing the radial distance betweenturns only by the angle mr where n is an even integer of either sign.This is expressed by the equation where r is the average radius of aspiral turn, g is the phase constant of the waveguide, ,6e is the phaseconstant of the beam modulation wave and h is the increment in thespiral radius in one turn, these terms being further defined as follows:

RWE-

Substituting the above value of r in (2) and rearranging The phaseconstant of a rectangular waveguide in terms of its greater dimension ais where is the free space wavelength of the electromagnetic wave. Usingthe relationships 21r and c 21rc )WFT when is the free space phaseconstant of the wave and c is the velocity of light, Equation 4 becomes5 fylrgy Equating (3) and (S) 1rC 2 (Lfmjtl'ba) Solving (5) for a givesh Heem Since, as indicated above, the value `of r in Equation 2 is theaverage radius of the spiral turn ending at the spiral angle 0, thevalue of a given by Equation 7 is that value of a required to give theproper phase shift in one complete turn of a rectangular waveguide ofconstant cross section formed in a circle of radius r. However, in orderto obtain the proper phase relations along all radii of the disc beam,the cross section of the spiral waveguide in FIGS. 1-2 cannot remainconstant in any given turn of the spiral but must continuously decreasethroughout the spiral as 0 increases. Therefore, the value of a computedfrom Equation 7 is an effective value of a for the spiral turn ending at0, with the actual value of a being greater than the effective value inthe earlier part of the turn and less in the latter part. Assuming theactual and effective values to become substantially equal midway of theturn, the actual value of a at any point 0 in the spiral, measured fromline 27 in FIG. 2, may be determined by substituting 0=0+vr in Equation7. Thus, to determine the values of a at the points where line 27intersects the spiral waveguide (0=0, 2r, 41r, 61r, etc.) the values of6 to be substituted in equation 7 are 1r, 3f, 51r, 71r etc. By drawing asmooth curve through these values of a plotted against 0', the value ofa at any point 0' in the spiral may be determined. From this informationa spiral rectangular waveguide having the proper taper of its longerdimension may be constructed.

It will be obvious to those skilled in the art that the positions ofcathode 14 and collector 21 may be interchanged. This would not requireany change in the construction of the waveguide.

I claim:

A traveling-wave tube comprising: means including an annular cathode andan annular collector electrode for producing a thin disc-shaped beam ofelectrons between said cathode and collector; a rectangular waveguidewound in a linear spiral and situated between said cathode and saidcollector, said spiral waveguide being concentric with said disc-shapedbeam and symmetrically positioned with respect to the central plane ofsaid beam, the larger dimension of said waveguide being normal to saidcentral i plane and said waveguide being slit at the center of itslarger dimension to permit passage of said beam; means for introducingan electromagnetic wave at one end of said spiral waveguide and'forremoving said electromagnetic wave from the other end of said spiralwaveguide after propagation therethrough, said propagated waveinteracting with said beam to produce a velocity modulation of itselectrons and a resulting density modulation of the beam; and saidspiral waveguide further having its larger dimension vary continuouslyas an inverse function of the spiral angle such that the differencebetween the phase shift incurred by said electromagnetic wave intraveling around any complete turn of said spiral waveguide and thephase shift incurred by the density modulation of said beam in travelingthe radial distance between turns of said spiral waveguide issubstantially equal to mr radians where n is an even integer.

References Cited by the Examiner UNITED STATES PATENTS 2,654,004 9/1953Bailey 315--4 X HERMAN KARL SAALBACH, Primary Examiner.

S. CHATMON, J R. Assistant Examiner,

